To comply with the new anti-pollution standards, which impose more restrictive exhaust gas emission limits with respect to the existent, it is known the use of the so-called purification system by selective catalytic reduction, also known under the acronymic SCR (“Selective Catalytic Reduction”) of the exhaust-gases of a vehicle. Such system is adopted, in particular, for reducing the emission into the atmosphere of nitrogen oxides.
Preferably, the SCR system is used in light-duty and heavy-duty vehicles.
The SCR system consists of injecting by compressed air at the catalyser inlet an aqueous solution of urea or equivalent product capable of reacting with the nitrogen oxides. Generally, it is preferred an aqueous solution of 32.5% of urea, for example the solution commonly marketed by Basf under the name AdBlue. The catalyser is arranged at the exhaust gas muffler and, inside it, the introduced urea reacts with the nitrogen oxides, also commonly called NOX, eliminating them from the atmospheric emissions.
The aqueous urea solution is generally contained in a tank from where it is taken to be injected into the catalyser, after being mixed to compressed air, according to times and modes defined by an electronic control unit which accounts for various parameters such as for example temperature, humidity, engine operation and number of revolutions.
A known problem of the SCR system is that during the parking of vehicles at temperatures lower than −12° C. the urea solution freezes. For this reason when the engine is ignited after parking, the pipings containing the urea solution are immediately heated up by passage of electrical current.
For example, it is felt the need to reach a temperature of at least +5° C. for the urea solution at least ten minutes after the engine is ignited, with the SCR system initially placed at an ambient temperature of −35° C.
Furthermore, the pipings which convey the aqueous urea solution must allow the passage of fluid in the working temperature range, that is from −40° C. to 80° C., must be flexible and must ensure that, in −40° C. conditions, the aqueous urea solution does not freeze in any part in all flow rate conditions, that is from 0 to 5.5 l/h.
For heating up the aqueous solution there are therefore generally used electrically heated pipings and pipe fittings.
For example, it is known the use of heated pipings comprising TEFLON® fluid conveying pipes, with an electrical resistance externally wound on a polyethylene heat-shrink sheath which also has the function of electrical insulation and is further covered by a corrugated polyamide tube.
Such solution however does not allow to obtain a sufficiently rapid defrosting of the aqueous solution to respond to the aforementioned requirements due to the fact that the TEFLON® does not allow sufficient conduction of heat to warm up the inside of the pipe. The TEFLON® pipes also present problems in use in coupling of couplings and are also very costly.
Such pipes also present the defect of presenting a high dissipation of the heat produced by the electrical resistor. The generated heat is indeed only partially supplied to the mass of solid solution to be heated because a portion is dispersed between the heater and the external environment at −35° C.
Furthermore, the known solutions described above are not very satisfactory, because they cannot always achieve a need to eliminate “plugs” of solid solution which are formed inside the connections of the heated pipings.
Indeed, the heat generated by the electrical resistors spirally wound on the pipings cannot always sufficiently warm up the solidified part of solution inside the pipe connections.